1. Field of the Invention
The invention relates generally to magnetoresistive sensors or read heads for reading magnetically recorded data, and more particularly to a method for making such heads.
2. Description of the Related Art
In magnetic recording disk drives the conventional sensor for reading the data recorded on the magnetic medium is a magnetoresistive “spin-valve” (SV) read head. The SV read head is a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically-conductive spacer layer. One ferromagnetic layer, typically the bottom ferromagnetic layer closer to the read head substrate, has its magnetization direction fixed or pinned, typically by being exchange coupled with an underlying antiferromagnetic layer. The other ferromagnetic layer is the “sensing” layer that has its magnetization direction “free” to rotate in the presence of the external magnetic field from the recorded data. The free layer is the sensing portion of the read head because when a sense current is applied to the head, the rotation of the free-layer magnetization relative to the pinned-layer magnetization is detected or sensed as a change in electrical resistance. This is because the scattering of the electrons shared by the free and pinned layers is dependent on the orientation of their electron spin relative to the magnetization direction of the layer they penetrate.
The SV magnetoresistive read head used in all current magnetic recording hard disk drives operates with the sense current directed parallel to the planes of the layers in the stack, so it is referred to as a current-in-the-plane (CIP) read head. In a disk drive CIP-SV read head, the magnetization of the pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance.
Magnetoresistive read heads have been proposed that operate with the sense current perpendicular to the planes (CPP) of the layers in the stack. A CPP-SV read head is described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1):84-88 Part 1 January 2002. Another type of CPP read head is a magnetic tunnel junction (MTJ) read head in which the nonmagnetic spacer layer is a very thin electrically insulating nonmagnetic tunnel barrier. In a MTJ read head the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the free and pinned layers. While in a MTJ magnetoresistive read head the spacer layer is electrically insulating and is typically alumina (Al2O3), in a CPP-SV magnetoresistive read head the spacer layer is electrically conductive and is typically copper. A MTJ read head is described in U.S. Pat. No. 5,729,410.
Both CIP and CPP magnetoresistive read heads are formed by successive deposition of the layers on a substrate. The substrate is typically a ceramic or silicon wafer with a layer of electrically insulating material that serves as the first read gap layer if the head is a CIP read head, or with a layer of magnetic material such as permalloy that serves as the first magnetic shield layer if the head is a CPP read head. The stack of layers typically includes a seed layer on the substrate, an antiferromagnetic layer on the seed layer, the pinned ferromagnetic layer on the antiferromagnetic layer, the spacer layer on the pinned layer, the free ferromagnetic layer on the spacer layer and a protective capping layer on the free layer. Following the successive deposition of the layers to form the stack, a series of lithographic patterning and ion milling (or reactive ion etching) steps is performed to define the track width of the read head. The substrate is then lapped along a plane perpendicular to the substrate and parallel to the track width, which then defines the stripe height of the read head. Thus in the completed read head both the free layer and the pinned layer (as well as the underlying antiferromagnetic layer) have the same track width and stripe height dimensions.
One of the problems in magnetoresistive read heads occurs as the heads are made smaller, which is necessary as the track density and areal density of disk drives increase. As the track width and stripe height dimensions of the free layer and thus the pinned layer are decreased, the magnetic stability of the pinned layer is also decreased. The stability of the pinned layer, i.e., its ability to maintain its magnetic moment in the desired pinned direction, decreases with decreasing volume of ferromagnetic material making up the pinned layer. Thus as the track width and stripe height dimensions decrease, the volume of the pinned layer, and thus its magnetic stability, also decreases.
Published patent application US 2003/0231437 A1 describes a CPP read head that has a pinned layer and underlying antiferromagnetic layer with a width greater than the free layer stripe height and a track width greater than the free layer track width. This CPP sensor is manufactured by conventional ion milling of the read head stack followed by selective oxidation of a region of the free layer, with the un-oxidized region of the free layer remaining as the active sensing region.
What is needed is a method for making a magnetoresistive read head that allows the width of the pinned layer and its underlying antiferromagnetic layer to be made larger than the stripe height of the free layer so that the sensing portion of the read head can be reduced without sacrificing the magnetic stability of the pinned layer.